The present invention relates generally to overhead cam air-cooled internal combustion engines, and more particularly to such engines wherein the camshaft is driven by an external timing belt and cam pulley.
Air-cooled internal combustion engines often employ a combination fan/flywheel mounted directly on the crankshaft externally of the crankcase to supply a flow of cooling air over the engine. Since the flywheel is located near the crankcase rather than near the cylinder and cylinder head, which are the portions of the engine most in need of cooling, a blower housing or other ducting is often used to direct the flow of air from the fan/flywheel toward and over the cylinder and cylinder head. This scheme works well in L-head or overhead valve type engines where the valve actuating mechanism is enclosed within the engine castings, since the exteriors of the cylinder and the cylinder head are unobstructed and covered with cooling fins.
In the case of toothed belt driven overhead cam type engines, however, a cam pulley is normally located on an extension of the camshaft externally of the cylinder head, and tends to obstruct the flow of air over the cylinder and cylinder head. This problem cannot be alleviated to a sufficient degree by reducing the diameter of the cam pulley, since four-cycle internal combustion engines require a 2:1 speed reduction of the camshaft relative to the crankshaft, which necessitates the cam pulley being twice as large as the drive pulley.
It has been found that even non-solid cam pulleys having a spoked support structure cause significant obstruction of air flow. This is believed to be caused by such pulleys acting as centrifugal blowers wherein the spokes tend to cause air to be thrown radially outwardly rather than passing through the open areas of the pulley.
It would be desirable to provide an overhead cam type air-cooled internal combustion engine with external drive of the camshaft while avoiding obstruction of cooling air flow over the cylinder and cylinder head.